html window cambiar texto selecionado

Gabino Oliver <[email protected]>
Newsgroups gmane.comp.python.general.castellano
Message-ID <[email protected]>
hola,
estoy usando html windows que carga y muestra texto basico
como puedo cambair el texto selecionado

por ejemplo seleciono:
hola mundo
y modificarlo por:
<div align="center">hola mundo </div>
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